Insulation consisting of ethylene-propylene rubber composition for electric wire and cable

ABSTRACT

Insulation material for electric wire and electric cable consisting of composition containing (a) ethylene-propylene rubber, (b) finely divided calcium carbonate surface-treated with hydrophobic fatty acid or its salt in amount of 10-80% by weight based on the ethylene-propylene rubber, and (c) heavy calcium carbonate in amount of 10-150% by weight based on the ethylenepropylene rubber and/or carbon black having particle size of at least 100 Mu in amount of 1-80% by weight based on the ethylene-propylene rubber.

United States Patent [72] Inventors Setsuya Isshiki Funabashi-shi; KaoruSaito, Tokyo; Hideo Sunazuka,

Tokyo, all of Japan [21] Appl. No. 643,931

[22] Filed June 6,1967

[45] Patented Nov. 2, 1971 [73] Assignee The Fujikura Cable WorksLimited Tokyo, Japan [32] Priority June 10, 1966 [33] Japan [54]INSULATION CONSISTING OF ETHYLENE- PROPYLENE RUBBER COMPOSITION FORELECTRIC WIRE AND CABLE 2 Claims, l Drawing Fig.

[52] U.S.C1 117/224, 1 17/226, 117/232 [51] Int. Cl H01b 3/30 [50] Fieldof Search 1 17/224, 232; 260/23 H, 27

[56] References Cited UNITED STATES PATENTS 2,927,091 3/1960 Lggett260/27 3,160,598 12/1964 Delfosse 260/23 H 3,362,924 1/1968 Eastman117/232 Primary Examiner*William D. Martin Assistant Examner-Raymond M.Speer Anarney-Wenderoth, Lind & Ponack ABSTRACT: Insulation material forelectric wire and electric cable consisting of composition containing(a) ethylenepropylene rubber, (b) finely divided calcium carbonatesurface-treated with hydrophobic fatty acid or its salt in amount of10-80% by weight based on the ethylene-propylene rubber, and (c) heavycalcium carbonate in amount of 10-150% by weight based on theethylene-propylenc rubber and/or carbon black having particle size of atleast 100 p. in amount of l80% by weight based on the ethylene-propyIenerubber.

INSULATION CONSISTING OF ETIIYLENE-PROPYLENE RUBBER COMPOSITION FORELECTRICWIRE AND CABLE This invention relates to an insulationconsisting of an ethylene-propylene rubber composition for wire andcable, wire and cable having a coating layer of said insulation and aprocess for the production thereof, particularly to an insulationmaterial consisting of an ethylene-propylene rubber composition havingpreferable combination of excellent electric characteristics andmechanical properties.

Ethylene-propylene rubber, for instance, an ethylenepropylene copolymeror an ethylene-propylene terpolymer, is known as an insulation havingvery excellent electric characteristics per se and is broadly used as aninsulating coating layer for electric power cable and other electricwire and cable.

Heretofore, a so-called EPR-insulated electric wire or cable having suchethylene-propylene rubber as an insulation has been obtained bysufciently kneading a composition of ethylene-propylene rubberincorporating therein a tiller such as clay, calcined clay or carbonblack together with a vulcanizing agent and other compounding agent,thereafter coating the outside of a conductor with the compounded rubbercornposition by either the extrusion or the covering method, andvulcanizing the coating.

However, when a relatively large amount of said filler was mixed inethylene-propylene rubber, the electric characteristics of the rubbercomposition remarkably lowered, while on the other hand, when the amountof the filler in ethylenepropylene rubber was limited to such amount asmight not bring remarkable lowering of the electric characteristics, ithas been unable to cause the mechanical strength of the insulating layerconsisting of the rubber composition to reach a sufficientlysatisfactory level. That is, a known EPR-insulated electric cable hasnot been satisfactory yet in combination of the electric characteristicsand the mechanical strength of the insulating coating layer.

Further in the field of the electric cable industry, it is alwaysdesirable to increase the amount of the filler in the insulating layeras much as possible thereby reducing the production cost of the electriccables.

An object of this invention is to provide an insulation consisting of anethylene-propylene rubber composition for electric wire and cable havingpreferable combination of excellent electric characteristics andmechanical properties and electric wire and cable having said insulationas a coating layer outside a conductor.

Another object of this invention is to provide an insulation material ofan ethylene-propylene rubber composition capable of compounding a largeamount of a filler without lowering the electric characteristics of thecomposition, accordingly, the cost for the production thereof isrelatively inexpensive.

Other objects and advantages of this invention will become clear fromthe following description.

The aforesaid objects are achieved by an insulation material of avulcanized ethylene-propylene rubber composition containing (a)ethylene-propylene rubber, (b) finely divided calcium carbonate surfacetreated by a hydrophobic fatty acid or its salt in an amount of -80parts by weight based on 100 parts by weight of the ethylenepropylenerubber, and (c) at least one kind of filler selected from the groupconsisting of heavy calcium carbonate and carbon black whose particlesize is at least 100 y., where in the case of said calcium carbonate, itexists in an amount of 10-150 parts by weight based on 100 parts byweight of the ethylene-propylene rubber, and in the case of said carbonblack, it exists in an amount of l-80 parts by weight based on l00 partsby weight of the ethylenepropylene rubber.

Ethylene-propylene rubber used in this invention is what is well knownas ethylene-propylene copolymer and ethylenepropylene terpolymer withthe characteristics of excellent electric properties, ozone resistanceand toughness. ln this invention, it is preferable to use anethylene-propylene copolymer consisting of -80 percent of ethylene and80-20 percent of propylene and an ethylene-propylene terpolymerconsisting of 20-80 percent of ethylene, 80-20 percent of propylene and0. l-l0 percent of a diolefin such as hexadiene.

The important characteristics of this invention resides in, as will bementioned in detail later, strict selection of the kind of fillercompounded and strict definition of the adding amount of each filler.

A first tiller used in this invention is calcium carbonate surfacetreated by a hydrophobic fatty acid or its salt.

Said hydrophobic fatty acid as referred to in this invention means ahigher aliphatic carboxylic acid, preferably saturated and unsaturatedhigher aliphatic monocarboxylic acids. Of these fatty acids, rhodinicacid, palmitic acid, stearic acid and oleic acid are preferable fortheir ready availability. And as a salt of a hydrophobic fatty acid, asodium salt, a potassium salt and a calcium salt may be used.

Calcium carbonate used may be either light calcium carbonate or heavycalcium carbonate, however, calcium carbonate whose particle size isbelow 200 y. after surface treated with arid fatty acid or its salt ispreferable from the viewpoint of electric insulation characteristics andweatherability of the ethylene-propylene rubber.

For treating the surface of calcium carbonate with said fatty acid orits salt, conventional means for forming a film on the surface of asolid may be adopted. For instance, finely divided heavy calciumcarbonate is immersed in a molten solution or a nonmolten liquidsolution or dispersion of calcium salt of a fatty acid. lt is alsopossible to produce the coated calcium carbonate particles by mixing afatty acid or its salt, for instance, sodium stearate, with lime milk(Ca(0H)) solution and blowing carbon dioxide gas through the mixture toform light calcium carbonate coated with calcium stearate. It isespecially preferable that the calcium carbonate particles have thecalcium salt of a fatty acid as the desirable film on the surface ofsaid particles. The amount of a fatty acid or its salt to be coated onthe surface of calcium carbonate is preferably 0.01-5 .0 percent byweight based on calcium carbonate,

The surface-treated calcium carbonate has not only interface propertiestending to uniformly disperse in ethylenepropylene rubber due to thesurface treatment with a fatty acid or its salt, but also calciumcarbonate surfaces treated with a fatty acid produced by the method ofblowing carbon dioxide in lime milk containing a fatty acid such as, forinstance, stearic acid produces a finely divided particle having aspherical configuration and a particle size of below 200 p which is thepreferable configuration for dispersing.

When electric characteristics of said surface-treated calcium carbonateused in this invention and fillers heretofore used as insulations forethylene-propylene rubber are compared, they become as shown in thefollowing table l.

From above table l, its is understood that volume resistivities of thesefillers are considerably inferior as compared with volume resistivity ofethylene-propylene rubber per se of LOXIO" (l. cm., however, thesurface-treated calcium carbonate in this invention is remarkablyexcellent in volume resistivity and tan. 8 as compared with the fillersuch as clay and talc, especially as compared with nontreated lightcalcium carbonate the volume resistivity is more than 1,000 times.

Next, these fillers are compounded with ethylene'propylene rubber withother compounding agents at a ratio shown below, and electriccharacteristics of said rubber compound vulcanized at 150 C. for 40minutes are shown by values of tan. (percent) in table 2 and mechanicalstrengths are shown by tensile strength (kg/mm?) in table 3.

Compounding Ratio (part by weight) Ethylenc-propylcne copolymer 100 Zincoxide 5 Paraff'ln I Filler Amounts shown in tables 2 and 3 Dicumylperoxide 3 Sulfur 2 Y TABLE- Tan. of ethylene-propylene rubbercomposition Compounding amount Kind of filler 20 50 70 l0() l50 Calciumcarbonate surface treated 0.5 0.5 0.5 0.7 1.9 3.4 with a fatty acid inthis invention Clay 0.5 L8 2.9 3.3 4.4 5.1 Tale 0.5 0.9 1.2 l.6 2.2 3.2Heavy calcium carbonate 0.5 0.5 0.5 0.55 0.7 1.0 Carbon black having aparticle 0.5 0.5 0.6 |.0 2.4 4.0 size of at least |00 p. Diatomaceousearth 0.5 I.0 1.7 2.1 2.9 3.8 Magnesium carbonate 0.5 |.9 3.| 4.0 5.23.9

TABLE 3 Tensile strength of ethylene-propylene rubber composition(kg/mm?) The present inventors have found that when the value of tan. ofethylene-propylene rubber composition is limited to a value lower than lpercent, an insulation material consisting of such ethylene-propylenerubber composition is sufficiently satisfactory as wire and cableinsulation. Thus, from said table 2 and table 3, it is understood thatby compounding lO-80 parts, preferable 40-70 parts by weight based on100 parts by weight of the ethylene-propylene rubber of saidsurfacetreated calcium carbonate in said rubber, it is possible toremarkably improve the mechanical strength without lowering the electriccharacteristics of the vulcanized rubber composition. Namely, when thecompounding amount exceeds 80 parts by weight, as shown in said table 2the electric characteristics, for instance, a value of dielectric losstangent (tan) becomes worse, on the other hand, when the compoundingamount of said surface-treated calcium carbonate is less than l0 partsby weight, an insulation having the desired mechanical strength cannotbe obtained and the object of reducing the cost of an insulation bycompounding of a filler cannot be achieved.

A second filler used in this invention is heavy calcium carbonate and/orcarbon black having an average particle size of at least y.. Heavycalcium carbonate used in this invention is well known as heavy calciumcarbonate or natural Whiting, and generally itis preferable to use whathas specific gravity of 2.662.77 and an average particle size of0.0l-l0y..

On the other hand, carbon black usable in this invention is that havingan average particle size of at least 100 y., especially preferably anaverage particle size within the range of 100-200 p.. Thus, in thisinvention carbon black produced by a furnace process such as finethermal (F.T) and semireinforcing furnace (S.R.F.) can be used.

Use of a carbon black such as channel black whose range of particle sizeis in colloidal range is undesirable because such use results inreducing the volume resistivity of an insulating layer consisting of theethylene-propylene rubber composition.

The compounding amount of a second filler of said group used in theprocess of this application varies according to the kind offtller,however, as shown in said table 2 such amount as may not cause tan. ofthe ethylene-propylene rubber com position to exceed l percent is used.

ln the case of using heavy calcium carbonate, it is compounded in anamount of l0l50 parts, preferably l0-5O parts by weight based on l0()parts by weight of the ethylenepropylene rubber, on the other hand, whencarbon black is used it is compounded in an amount of l-80 parts,preferably 5-30 parts by weight based on 100 parts by weight of saidrubber. The compounding amounts of these fillers do not nor mally haveto be changed even when the two fillers are used concurrently.

Thus, calcium carbonate surface treated by a fatty acid or its salt,heavy calcium carbonate and/or carbon black are compounded inethylene-propylene rubber in accordance with conventional methodtogether with a cross-linking agent, vulcanization accelerator, softenerand aging resistor.

ln the case of an ethylene-propylene copolymer, as a known cross-linkingagent peroxide such as for instance, dicumyl peroxide, benzoyl peroxideand tert-butyl benzoyl peroxide is employed and as a vulcanizationaccelerator, quinone dioxime, dibenzoyl quinone` dioxime and diarylphthalate are employed.

ln the case of an ethylene-propylene terpolymer, a cross linking agentsuch as peroxide, sulfur, quinone dioxime and thiurum type cross-linkingagent; a vulcanizaton accelerator such as quinone dioxime,benzoylquinone dioxime and thiurum type vulcanization accelerator; asoftener such as naphthene, paraffinand aromatic-type hydrocarbon; andan aging resistor such as 4,4'-thiobis- (6-tert-butyl-3-methyl) phenoland polymerized trimethyl dihydroquinoline are respectively employed andwhen desired other compounding agents such as color pigment may beemployed.

As the compounding method, a method of mixing a filler in ordinaryrubber may be adopted, namely, by using a known apparatus such as rollsor Bumburys mixer kneading operations may be carried out.

The compounded rubber having finished kneading operaY tions is coated onthe outside of a conductor by the extrusion method or the coveringmethod thereafter the coating of said rubber is pressingly wound with acotton tape and vulcanized according to the conventional process. Theconditions of the vulcanization vary depending upon the vulcanizingagent and the vulcanization accelerator used, however, generally thevulcanization may be easily carried out by heating the compoundedcomposition at 200 C. for l minute to 3 hours.

ln this case, it is normal to coat the rubber composition immediatelyabove a conductor in the case of a small capacity wire and cable, and tocoat the rubber composition via a semiconductive layer such as carbonpaper or semiconductive rubber in the case of a large capacity cable.

Thus, the insulated electric wire of this invention is usable per se forvarious uses. The insulated-core wire so obtained is usually provided onits exterior with a protective covering for example polyvinyl chloride,chloroprene or lead sheathing, directly in the case of a single corecable, but after twisting together with other interposing materials inthe case of a multicore cable. ln the case of large-capacity cables, theforegoing protective covering is provided to the exterior of the cable,which has a metallic shielding tape, e.g. of copper or brass, appliedabout each individual insulated core wire or about an assembly of corewires which have been twisted together.

The FIGURE is a sectional view showing sectional structure of oneembodiment of the ethylene-propylene rubber-coated electric cable,namely, a high-voltage single conductor cable.

In the FIGURE l is a conductor, 2 is a semiconductive layer, 3 is aninsulating layer of ethylene-propylene rubber compounded with calciumcarbonate surface treated by a hydrophobic fatty acid, and heavy calciumcarbonate and/or carbon black, 4 is a semiconductive layer, 5 is ashield tape layer and 6 is a polychloroprene sheath. This inventioncovers not only a case wherein the insulating layer 3 in a power cableconsists of a single layer of said elthylene-propylene rubbercomposition only, but also a case wherein said layer 3 consists of acomposite insulating layer of a layer of said ethylenepropylene rubbercomposition and a layer of another synthetic rubber composition. ln thecase of the latter, insulation become graded insulation, becoming anelectrically excellent structure.

The ethylene-propylene rubber composition compounded withsurface-treated calcium carbonate, and heavy calcium carbonate and/orcarbon black as fillers at specific volume ratio is remarkably excellentin combination of electric characteristics and mechanical strength ascompared with a known ethylene-propylene rubber composition compoundedwith as fillers calcined clay, talc, clay, calcium carbonate andmagnesium carbonate.

Heretofore it has been known to compound ethylenepropylene rubber withvarious fillers as insulation material for electric wire and cable,however, the filler capable of improving mechanical strength of anethylene-propylene rubber composition normally has been tending to lowerthe electric characteristics of the insulation material and a fillersatisfying both the mechanical strengths and the electrical propertiesof an ethylene-propylene rubber composition has not been found.

Whereas, the present inventors have found that by selecting calciumcarbonate surface treated with a hydrophobic fatty acid or its saltexcellent in electric characteristics as a filler per se as comparedwith other conventional fillers, at the same time, showing excellentmechanical strengths when cornpounded in rubber, compounding the samewith one or both of heavy calcium carbonate or carbon black having aparticle size of at least 100 it, and defining the amounts of thesefillers such that when each of these fillers is singly compounded inrubber, the value of tan. may not exceed l percent, the obtainedinsulation material consisting of an ethylene-propylene rubbercomposition has combination of excellent electric characteristicsrepresented as shown in undermentioned examples by tan, smaller than lpercent and excellent mechanical strengths.

lt is heretofore accepted common sense that when content of a filler inrubber is increased, the electric characteristics of the rubbercomposition lowers as the compounding amount increases. Therefore, it isanticipated that use of at least two fillers would, as the total amountof these fillers increases result in proportional lowering of theelectric characteristics of the rubber compound. Whereas, unexpectedlyin this invention despite use of two or three kinds of filler, by makingthe compounding amount of each of said fillers such that when each ofsaid fillers is singly compounded in rubber the value of tan. may notexceed l percent, an ethylene-propylene rubber composition compoundingtwo or three kinds of filler therein is maintained in such a range thatvalue of tan. may not exceed l percent.

Further, the ethylene-propylene rubber composition of this inventionbesides having said preferable combination of electric characteristicsand mechanical strengths, due to use of said surface-treated calciumcarbonate, has an advantage that lowering of electric characteristicseven when exposed under hydroscopic conditions for a long period islittle. Further, when in accordance with this invention, carbon blackwhose average particle size is at least l0() ,L is used concurrently inan amount within the range of lparts by weight, there is an advantagethat weatherability of the insulating layer is im proved, further,imparting tracking resistance thereto without lowering dielectricstrength.

This invention will be explained by the following example.

EXAMPLE l ln ethylene-propylene rubber (consisting of 50 mols ofethylene and 50 mols of propylene, degree of polymerization: 150,000,trade name: Dutral), together with the undermentioned compoundingagents, light calcium carbonate having a coating of calcium stearate andheavy calcium carbonate were compounded in various amounts shown intable 4 for 0.3 hours by rollers. The compounded rubbers were formedinto sheets l mm. thick and 20 cm. X 20 cm. dimension by using a press,followed by subjecting these sheets to steam vulcanization at 150 C. for40 minutes, The electric and physical characteristics of thesevulcanized rubbers were as follows.

Compounding Ratio (parts by weight) Ethylcnc-propylcnc rubber |00 Zincoxide 5 Paraffine l Surfacmtreated light Amounts shown in calciumcarbonate table 4 below Dicumyl peroxide 3 Ouinone dioxime 2 Heavycalcium carbonate Amounts Shown in table 4 below TABLE 4 PrescriptionItem A B C D Compounding amount:

Surfacetreated light calcium carbonate. 80 80 80 100 Heavy calciumcarbonate... 150 200 50 Electric characteristics:

Tan. (percent) room temp. 0. 60 1. 00 2. 20 1. 90 Volume resistivity (Slem.) 7.1)(105 5X105 0x10 2Xl0l5 (K. mm.) dielectric strength 35 30 28 31Physical characteristics:

Tensile strength (kg/mm!) 0. 8i 0. 78 0. 62 0.83 Elongation (percent)750 700 580 900 For information, in the above example when instead ofcalcium carbonate surface treated with stearic acid, calcium carbonatesurface treated with rhodinic acid, calcium carbonate surface treatedwith palmitic acid and calcium carbonate surface treated with oleic acidwere used, the similar results were obtained.

From the results of above table 4, it is understood that by maintainingthe respective compounding amounts of surfacetreated calcium carbonateand heavy calcium carbonate such that when each of these calciumcarbonates is singly compounded, value of tan. may not exceed l percent(Prescriptions A and B), it is possible to improve the mechanicalstrengths ofthe insulation material and maintain value of tan. less thanl percent, on the other hand by the prescriptions outside the range ofthis invention (Prescriptions C and D), lowering of the electriccharacteristics is remarkable.

ethylene and 50 mols of propylene, degree of polymerization: 150,000,trade name: Dutral), besides the undermentioned Compounding Rullo (partsby weight) Fthylcnwpropylcnc rubber 100 Zinc oxide 5 Parafn l Heavycalcium carbonate AmOUntS 21S Shown ln 5 table 6 below compoundingagents, light calcium carbonate having a coating surfawoaicti iightAmounts as shown of calcium rosinate and carbon black (fine thermal)whose Calcium Carbonate [TMW f' hvlw Carbon black having a mounts :is sown average particle size. being at least 100 p. were added innvariparticle me or m0 m in able b bclnw ous amounts shown in table 5below and compounded in mix- Dicumyl peroxide 3 ing rolls for 0.3 hour.Using press, these compounded rubbers l() Sulfur 2 TABLE u ItemPrescription J I( L M N Surface-treated light calcium carbonate 8U 10040 40 40 50 Compounding amount Heavy calcium carbonate 40 40 50 50 150lo() Carbon black having particle Size of 100 Inu 50 5U 5 0 K 100 10 5UTan, (percent) room temp 0.81 2. 0. b5 .2. 50 0. mi I 1.5i) Electriccharacteristics {Volumc resistivity (S2 cm.) room tcnip GX10la 1 10l58.5X1Q15 0. U `ttll 3X10 5x19? Dielectric strength (kv./iii1n.) 30 28 334 g1 :A Tensile strength (kga/mm!) 0. 85 0, .10 0. 80 0. iti (l.- i5 0,i() Physlcal chumcmustics "{Eloiigatioii (percent) 750 78() U00 750 i0()(301) were formed into sheets l mm. thick and cm. X 20 cm. dimension,subsequently the sheets were subjected to steam vulcanization at 150 C.for 40 minutes. The electric and physical characteristics of thisvulcanized rubbers were as follows.

Compounding Ratio (parts by weight) llhylencpropylene rubber 100 Zincoxide 5 Paraffin l Carbon black whose average Amounts shown in From theresults of above table 6, it is apparent that when three kinds ofcompounding agent of surface-treated calcium carbonate, heavy calciumcarbonate and carbon black are compounded by maintaining the compoundingamounts of the respective compounding agents such that when each of saidcompounding agents is singly compounded, value oftan. may not exceed lpercent (prescriptions J. L and N), it is possible to improve thephysical strengths ofthe insulation materials and maintain value of tan.below l percent. On the other particle size is ai least icc ma tablc 5below 30 hand, the rubber compounds wherein the compounding SlffwmcdCacm Cabofac 'lluusn hw l amount of each compounding agent is outsidethe range of this Dicumyl peroxide d e 3 c uw invention (prescriptionsK, M and O), considerable lowering Emmy, ummm ("ximc 2 of the electriccharacteristics is observed.

TABLE 5 Prescription Item E F G II l Compounding amount:

Carbon black having c particlo size of 100 mit 80 80 00 100 100Surface-treated light CaCO; 80 100 8O 100 Electric characteristics:

Tan. 6 (percent) room temp 0. 98 2. 30 2. 50

Volume resistivity (9cm.) at room temp.

(Kv./mrn.) dielectric strength Physical characteristics:

Tensile strength (kg/mm2) Elongation (percent) 0. 95 7X1015 7. 2X10l5 353G From the results of above table 5 it is apparent that by maintainingthe respective compounding amounts of surfacetreated calcium carbonateand carbon black such that when each of them is singly compounded, valueof tan. may not exceed l percent (prescription E and F), it is possibleto improve the physical strengths of the insulation materials andmaintain values of tan. below l percent. On the other hand, in the casesof the rubber compounds whose compounding amounts of fillers are outsidethe range of this invention (prescriptions G, H and I), lowering of theelectric characteristics is observed.

EXAMPLE 3 To ethylene-propylenc rubber (consisting of mols of ethyleneand 50 mols of propylene, degree of polymerization: 150,001, trade name:Dutral), besides the undermentioned compounding agents, light calciumcarbonate having a coating of calcium oleate, heavy calcium carbonateand carbon black (medium thermal) having an average particle size of atleast 100 u were added in amounts shown in table 6 below and compoundedin mixing rolls for 0.3 hour. The compounded rubbers were formed intosheets of l mm. thick and 20 cm. X 20 dimension using a press,subsequently the sheets were subjected to steam vulcanization at 150 C.for 40 minutes. The electric characteristics and physicalcharacteristics of these vulcanized rubbers were shown in table 6 below.

From the aforementioned results, it is understood that theethylene-propylene rubber composition of this invention wherein theamount of each filler is limited to the specific range (prescriptions A,B, E, F, J, L and N) has preferable combination of electriccharacteristics and physical strengths as an insulation material ascompared with the compositions whose amount of each filler is outsidethe range of this invention.

We claim:

l1. An ethylene-propylene rubber insulated electric wirc or cable whichcomprises an electric wire or cable coated with insulating layercomposed of an ethylene-propylene rubber composition consistingessentially of (a) an ethylenepropylene rubber; (b) 10-80 parts byweight, based on 100 parts by weight ofthe ethylencpropylene rubber, ofsurfacecoated finely divided calcium carbonate obtained by preparing amixture of lime milk, and a monovalent higher fatty acid or a sodium,potassium or calcium salt thereof and blowing carbon dioxide gas intosaid mixture, the amount of the surface coating being 0.0l-5.0 percentby weight based on the calcium carbonate; and (c) at least one fillerselected from the group consisting of heavy calcium carbonate having aspecific gravity of 2.66-2 77 and an average particle size of 0.01-10 )tand mixtures thereof with carbon black having a particle size 0f atleast 100 it, said heavy calcium carbonate being employed in an amountof 10-150 parts by weight based on 100 parts by weight of theethylene-propylene rubber and said carbon black being employed in anamount of l-80 parts by weight based on 100 parts by weight of theethyIene-propylene rubber.

2. A process for the production of an ethylene-propylene rubberinsulated electric wire or cable which comprises coating a conductor ora conductor having a semiconductive layer thereon with anethylene-propylene rubber composition consisting essentially of (a) anethylene-propylene rubber, (b) l-80 parts by weight, based on 100 partsby weight of the ethylene-propylene rubber, of surface-coated, finelydivided calcium carbonate obtained by preparing a mixture of lime milkand a monovalent higher fatty acid or a sodium, potassium or calciumsalt thereof and blowing carbon dioxide gas into said mixture, theamount of the surface coating being 0.0l-5 percent by weight based onthe calcium carbonate, (c) a vulcanizing agent, (d) a vulcanizingaccelerator and (e) at least one filler selected from the groupconsisting of heavy calcium carbonate having a specific gravity of2.66-2.77 and an average particle size of 0.()l-l0 p. and mixturesthereof with carbon black having a particle size of at least l00 u, saidheavy calcium carbonate being employed in an amount of 10-150 parts byweight based on |00 parts by weight ofthe ethylene-propylene rubber,said carbon black being employed in an amount of lparts by weight basedon 100 parts by weight of the ethylene-propylene rubber, and thenvulcanizing the coated rubber composition.

2. A process for the production of an ethylene-propylene rubberinsulated electric wire or cable which comprises coating a conductor ora conductor having a semiconductive layer thereon with anethylene-propylene rubber composition consisting essentially of (a) anethyleNe-propylene rubber, (b) 10-80 parts by weight, based on 100 partsby weight of the ethylene-propylene rubber, of surface-coated, finelydivided calcium carbonate obtained by preparing a mixture of lime milkand a monovalent higher fatty acid or a sodium, potassium or calciumsalt thereof and blowing carbon dioxide gas into said mixture, theamount of the surface coating being 0.01-5 percent by weight based onthe calcium carbonate, (c) a vulcanizing agent, (d) a vulcanizingaccelerator and (e) at least one filler selected from the groupconsisting of heavy calcium carbonate having a specific gravity of2.66-2.77 and an average particle size of 0.01-10 Mu and mixturesthereof with carbon black having a particle size of at least 100 Mu ,said heavy calcium carbonate being employed in an amount of 10-150 partsby weight based on 100 parts by weight of the ethylene-propylene rubber,said carbon black being employed in an amount of 1-80 parts by weightbased on 100 parts by weight of the ethylene-propylene rubber, and thenvulcanizing the coated rubber composition.